dsh-loan-prepay: Loan Prepayment Calculator

`dsh-loan-prepay` is a plugin in the DeepSeek Harness (DSH) ecosystem for financial planning and loan processing. It is a pure Node.js implementation with no network dependencies, capable of precisely calculating early repayment details based on specified parameters. Users can install it via a command and invoke its registered tools. Before use, environment dependencies must be verified, and the plugin runs with the permissions of the current process.